Dinoflagellate cysts occur extensively in the surface sediments of Lake Nakaumi, brackish-water lake in western Japan. The cyst assemblage is composed mainly of Brigantedinium spp., Algidasphaeridium? sp. aff. minutum, and other heterotrophic species. The assemblage is formed in a high productivity area, maintained by waters rich in dissolved nutrients from rivers. The change of the species composition in the lake is hardly observed. The species composition rate, however, indicates some differences in waters in the lake. The disparity is probably caused by an inflow of fresh water from some rivers around the lake, and may be directly influenced by salinity fluctuations.
Key words: brackish-water lake, dinoflagellate cyst, heterotrophic species, Lake Nakaumi, salinity